Triboelectric Nanogenerator (TENG) is a promising energy-harvesting device designed to generate electrical power through mechanical excitation caused by repeated contact and separation between suitable surfaces. During this process, tribocharges appear in the zones where the surfaces are in actual contact. Consequently, roughness and stiffness of the tribo-layers have a crucial effect on the TENG output performance. In particular, when a soft layer is pressed against a hard surface with large roughness, the real contact area becomes greater than the nominal one, resulting in improved performance [1]. However, experimental observation of the actual 3D contact zone between the layers for such scenario is complicated. The present study aims to develop an accurate finite element model of TENG, coupling contact mechanics and electrostatics while considering large slopes on surface roughness. For a given contact force, material properties, and surface roughness, the numerical solution presents the evolution of the real contact area and location of tribocharges on contacting surfaces. Furthermore, the time-dependent electrostatics analysis for the separation stage presents the evolution of the output voltage with changing air gap between layers. The results agree to an appropriate extent with analytical approximations and experimental observations. These advancements contribute to developing a unified multi-physical framework utilising MoFEM [2], aimed to reduce experimental efforts and facilitate circuit design and material optimization of TENG. REFERENCES [1] C. Kumar, J. Perris, S. Bairagi, G. Min, Y. Xu, N. Gadegaard, and D. M. Mulvihill, Multiscale in-situ quantification of the role of surface roughness and contact area using a novel Mica-PVS triboelectric nanogenerator. Nano Energy, Vol. 107, (2023) [2] L. Kaczmarczyk, Z. Ullah, K. Lewandowski, X. Meng, X.-Y. Zhou, I. Athanasiadis, H. Nguyen, C.-A. Chalons-Mouriesse, E. J. Richardson, E. Miur, A. Shvarts, M. Wakeni, and C. Pearce. MoFEM: An open-source, parallel finite element library. The Journal of Open Source Software, Vol. 5, (2020)